Porcelain enamel slip and method for preparing the same



United States Patent Ofitice PORCELAIN ENAMEL but AND METHOD FOR PREPARlNG TEE SAME Dean S. ubhell, Connersville, End, and Ernest P- Weaver, Pittsburgh, Pa, assigncrs to H. H. Robertson Company, Pittsburgh, Pa, a corporation of Pennsyl- Vania No Drawing. Original application Feb. 28, 1953, Ser. No. 718,176, now Patent No. 2332,58 1, dated Apr. 12, 1960. Divided and this application Sept. 22, 1959, Ser. No. 841,454

6 Ciaims. (Cl. 106-4?) The present invention relates to porcelain enamel slip preparing the same. More particularly the invention is concerned with a porcelain enamel slip having increased storage life.

This invention is especially concerned with the socalled low temperature enameling, i.e., those which 1nature at temperatures below about 1050 F.

The present invention is a division of our copending application S.N. 718,176, filed February 28, 1958, now U.S. Patent 2,932,584, which in turn is a continuationin-part of our copending application S.N. 542,238, filed October 24, 1955, which in turn is a continuation-in-part of an earlier filed application, now abandoned, S.N. 500,988, filed April 12, 1955.

In the porcelain enameling of metal surfaces, a suitably treated metal surface is coated with an aqueous slurry of glass frit to a uniform thickness. The aqueous slurry is termed an enamel slip. The slip usually contains from about 4.5 to 5.5 gallons of wator for each 100 pounds of frit. The metal surface, coated with slip, is dried to leave behind a residue of uniform thickness called 2. bisque which consists of the solid particles which had been suspended in the slip.

of glass on the metal surface.

titanium dioxide, antimony oxide and similar metal oxand the like are added to the porcelain enamel slip to offset the gradually increasing The porcelain enamel slips in the past have exhibited very poor storage properties. On standing for a matter of a week or so (and occasionally only for a few days) they become lumpy and finally harden into a solid mass. Accordingly it is the practice in the enameling industry to prepare porcelain enameling slips for use in the nearterm future. Color reproduction and color matching often becomes a trial-and-error problem in preparing porcelain enameling slips for reproducing previous production.

The object of the present invention is to prepare a porcelain enamel slip which has increased storage life. A further object of the invention is have discovered Patented Feb. 26, 1963 Hubbell, assigned to the assignee of the present invention. Such copper powders have been prepared having a mean particle diameter less than about 5 microns.

EXAMPLE I Two mill batches (one'gallon each) of porcelain enamel slip were prepared simultaneously. Each employed the same standard slip formulation.

Standard Slip Formulation 10 pounds frit (prepared according to process of U.S.

Patent 2,467,114, Deyrup) 3.37 ounces boric acid 6.75 ounces potassium silicate 4.0 ounces potassium hydroxide 1.0 pound titanium dioxide (TXA grade for matte) 8.0 ounces titanium dioxide (RA-1O grade foropacitying) 4.0 ounces cobalt compounds (for blue coloring) 5.0 pounds water One mill batch (A) contained 1.4 grams of cupreous powder as described in the aforementioned U.S. Patent 2,420,540. There were no additions to the second mill of grams left no mesh U.S. standard screen.

The slip without interground copper became hardened and could not be used after 2 /2 to 3 weeks. The slip containing the copper, after 11 months storage, could be easily stirred with a spoon to a sprayable consistency and produced a satisfactory porcelain enamel.

Frits, according to the aforesaid Deyrup 114, contain as essential ingredients, lead lithium oxide, and at least one other alkali metal oxide (PbO) The total alkali metal oxide content charges are comminu-ted by 3 EXAMPLE II A batch oi. porcelain enamel slip was prepared by admixing and milling the following ingredients:

100 pounds frit (according to U.S. Patent 2,467,114),

3.25 pounds boric acid,

6.50 pounds potassium silicate,

2.5 pounds potassium hydroxide,

13.5 pounds titanium dioxide (TXA grade, matte),

2.5 pounds titanium dioxide (RA-l grade, opacity),

0.5 pound antimony trioxide,

25 ounces color oxides (cobalt compounds), and

14 grams cupreous powder (prepared according to U.S.

Patent 2,420,540)

gallons water The batch was milled until no more than 0.2 gram out of a lOO-gram sample was retainedon a 325-mesh U.S. standard screen. The resulting porcelain enamel slip was stored at room temperature. The slip remained suitable tor immediate use after eleven months of storage. The copper content was 0.03 percent by weight of the frit.

EXAMPLE 111 Three mill batches (one-gallon each) of porcelain enamel slip were prepared from the formulation listed in Example I. To the first mill batch (C) no copper was added. To the second mill batch (D), 0.02 percent by weight of copper powder was added. To the third mill batch (B) 0.25 percent by weight of copper powder Was added.

All three batches were milled until no more than 0.2 gram of residue was retained from a 100-gram sample on a 325-mesh U.S. standard screen.

At the end of nine months, specimen (C) was completely hardened. In fact, hardening was exhibited within one month of storage.

Specimen (D) was observed to be fluid after a period of nine months. After 27 months storage, specimen (D) could be stirred, with some effort, into a smooth slip which produced a satisfactory porcelain enamel.

Specimen (E) remained fluid for several months but was becoming hard at the end of six months. Thus it appears, from specimen (E), that it is possible to intergrind an excessive amount of copper powder which will prevent the desired storage life extension.

Accordingly we prefer to add from 0.01 to 0.20 percent by weight (based on the weight of trit) of copper powder into the mill charge prior to comminution into trit.

The preferred method for introducing the copper powder is to combine the finely divided copper powder with the other ingredients of the frit prior to their comminution. By this technique the copper powder will be interground with the other ingredients. Normally the mill ball milling or rod milling to a consistency such that more than 98.2 percent will pass through a 325-mesh U.S. standard screen. By adding the friable, crystalline, copper powder, preferably having a mean particle diameter less than about five microns prior to milling, the copper particles are further reduced in size and intimately interground with the other ingredients.

According to the provisions of the patent statutes, we have explained the principle, preferred construction and mode of operation of our invention and have illustrated and described what We now consider to represent its best embodiment. However, we desire to have it understood that within the scope of the appended claims, the

invention may be practiced otherwise than as specifically illustrated and described.

We claim 1. An enamel slip comprising an aqueous slurry of 0.20 percent by weight oi finely divided crystalline copper particles, said slip having a greater storage life than an aqueous slurry of the said glass frit which contains no crystalline copper particles.

2. The enamel slip of claim 1 containing a metallic oxide opacifier.

3. The enamel slip of claim 1 wherein said copper particles have a mean diameter less than about 5 microns.

4. The enamel slip of claim 2 containing a pH buffering reagent.

5. In an enamel slip comprising an aqueous slurry of finely divided glass trit which comprises lead oxide, silica, lithium oxide and oxides selected from the group consisting of oxides of sodium, potassium and titanium, the improvement comprising a quantity of finely divided crystalline copper particles in said slip in an amount from 0.01 to 0.20 percent by weight of the said slip, whereby the resulting slip has a greater storage life than an aqueous slurry of the said glass frit which contains no crystalline copper particles.

6. The method of preparing an creased storage enamel slip having inlite comprising admixing glass particles which comprise lead oxide, lithium oxide and oxides selected from the glass consisting of oxides of sodium, potassium and titanium, together with 0.01 to 0.20 percent by weight of finely divided crystalline copper powder having a mean diameter less than about 5 microns, comminuting the said admixture until the said glass particles are reduced to frit, and admixing the comminuted admixture with sufficient water to form an aqueous slurry suitable as an enamel slip, whereby the resulting enamel slip has a greater storage life than an aqueous slurry of the said glass particles in comminuted form which contains no crystalline copper particles.

References Cited in the tile of this patent UNITED STATES PATENTS 1,693,825 Stoekle Dec. 4, 1928 

1. AN ENAMEL SLIP COMPRISING AN AQUEOUS SLURRY OF FINELY DIVIDED GLASS FRIT AND CONTAINING 0.01 TO 0.02 PERCENT BY WEIGHT OF FINELY DIVIDED CRYSTALLINE COPPER PARTICLES, SAID SLIP HAVING A GREATER STORAGE LIFE THAN AN AQUEOUS SLURRY OF THE SAID GLASS FRIT WHICH CONTAINS NO CRYSTALLINE COPPER PARTICLES. 